The invention relates to a foundry binder which is obtainable by surface modification of
a) colloidal inorganic particles with
b) one or more silanes of the general formula (I)
Rxxe2x80x94Sixe2x80x94A4xe2x88x92xxe2x80x83xe2x80x83(I)
where the radicals A are identical or different and are hydroxyl groups or groups which can be removed hydrolytically, except methoxy, the radicals R are identical or different and are groups which cannot be removed hydrolytically and x is 0, 1, 2 or 3, where xxe2x89xa71 in at least 50 mol % of the silanes;
under the conditions of the sol-gel process with a sub-stoichiometric amount of water, based on the hydrolysable groups which are present, with formation of a nanocomposite sol, and further hydrolysis and condensation of the nanocomposite sol, if desired, before it is brought into contact with the foundry sand.
The nanocomposite sol employed according to the invention is prepared by surface modification of colloidal inorganic particles (a) with one or more silanes (b), if desired in the presence of other additives (c) under the conditions of the sol-gel process.
Details of the sol-gel process are described in C. J. Brinker, G. W. Scherer: xe2x80x9cSol-Gel Sciencexe2x80x94The Physics and Chemistry of Sol-Gel-Processingxe2x80x9d, Academic Press, Boston, San Diego, New York, Sydney (1990) and in DE 1941191, DE 3719339, DE 4020316 and DE 4217432.
Here, specific examples of the silanes (b) which can be employed according to the invention and of their radicals A which are hydrolytically removable and their radicals R which are not hydrolytically removable are given.
Preferred examples of groups A which are removable hydrolytically are hydrogen, halogen (F, Cl, Br and I, in particular Cl and Br), alkoxy (in particular C2xe2x88x924-alkoxy, such as ethoxy, n-propoxy, isopropoxy and butoxy), aryloxy (in particular C6xe2x88x9210-aryloxy, such as phenoxy), alkaryloxy (e.g. benzyloxy), acyloxy (in particular C1-4-acyloxy, such as acetoxy and propionyloxy) and alkylcarbonyl (e.g. acetyl). Radicals A which are likewise suitable are amino groups (e.g. mono- or dialkyl-, -aryl- and -aralkylamino groups having the abovementioned alkyl, aryl and aralkyl radicals), amide groups (e.g. benzamido) and aldoxime or ketoxime groups. Two or three radicals A may also together form a moiety which complexes the Si atom, as for example in Si-polyol complexes derived from glycol, glycerol or pyrocatechol. Particularly preferred radicals A are C2xe2x88x924-alkoxy groups, in particular ethoxy. Methoxy groups are less suitable for the purposes of the invention, since they have an excessively high reactivity (short processing time of the nanocomposite sol).
The abovementioned hydrolysable groups A may, if desired, carry one or more usual substituents, for example halogen or alkoxy.
The radicals R which are not hydrolytically removable are preferably selected from the group consisting of alkyl (in particular C1xe2x88x924-alkyl, such as methyl, ethyl, propyl and butyl), alkenyl (in particular C2xe2x88x924-alkenyl, such as vinyl, 1-propenyl, 2-propenyl and butenyl), alkynyl (in particular C2xe2x88x924-alkynyl, such as acetylenyl and propargyl), aryl (in particular C6xe2x88x9210-aryl, such as phenyl and naphthyl) and the corresponding alkaryl and arylalkyl groups. These groups may also, if desired, have one or more usual substituents, for example halogen, alkoxy, hydroxy, amino or epoxide groups.
The abovementioned alkyl, alkenyl and alkynyl groups include the corresponding cyclic radicals, such as cyclopropyl, cyclopentyl and cyclohexyl.
Particularly preferred radicals R are substituted or unsubstituted C1-4-alkyl groups, in particular methyl and ethyl, and substituted or unsubstituted C6-10-alkyl groups, in particular phenyl.
It is also preferable that x in the above formula (I) is 0, 1 or 2, particularly preferably 0 or 1. It is also preferable if x=1 in at least 60 mol %, in particular at least 70 mol %, of the silanes of the formula (I). In particular cases, it may be even more favourable if x=1 in more than 80 mol %, or even more than 90 mol % (e.g. 100 mol %), of the silanes of the formula (I).
The foundry binder according to the present invention may be prepared, for example, from pure methyl-triethoxysilane (MTEOS) or from mixtures of MTEOS and tetraethoxysilane (TEOS), as component (b).
Concrete examples of silanes of the general formula (I) are compounds of the following formulae:
Si(OC2H5)4, Si(O-n- or iso-C3H7)4, Si(OC4H9)4, SiCl4, Si(OOCCH3)4, CH3xe2x80x94SiCl3, CH3xe2x80x94Si(OC2H5)3, C2H5xe2x80x94SiCl3, C2H5xe2x80x94Si(OC2H5)3, C3H7xe2x80x94(OC2H5)3, C6H5xe2x80x94Sixe2x80x94(OC2H5)3, C6H5xe2x80x94Si(OC2H5)3(C2H5O)3xe2x80x94Sixe2x80x94C3H6xe2x80x94Cl, (CH3)2SiCl2, (CH3)2Si(OC2H5)2(CH3)2Si(OH)2, (C6H5)2SiCl2, (C6H5)2Si(OC2H5)2, (C6H5)2Si(OC2H5)2, (iso-C3H7)3SiOH, CH2xe2x95x90CHxe2x80x94Si(OOCCH3)3, CH2xe2x95x90CHxe2x80x94SiCl3, CH2xe2x95x90CHxe2x80x94Si(OC2H5)3, HSiCl3, CH2xe2x95x90CHxe2x80x94Si(OC2H4OCH3)3, CH2xe2x95x90CHxe2x80x94CH2xe2x80x94Si(OC2H5)3, CH2xe2x95x90CHxe2x80x94CH2xe2x80x94Si(OOCCH3)3, CH2xe2x95x90C(CH3) COOxe2x80x94C3H7xe2x80x94Sixe2x80x94(OC2H5)3, CH2xe2x95x90C(CH3)xe2x80x94COOxe2x80x94C3H7xe2x80x94Si(OC2H5)3, n-C6H13xe2x80x94CH2xe2x80x94CH2xe2x80x94Si(OC2H5)3, n-C8H17xe2x80x94CH2xe2x80x94CH2xe2x80x94Si(OC2H5)3, 
These silanes can be prepared by known methods; cf. W. Noll, xe2x80x9cChemie und Technologie der Siliconexe2x80x9d [Chemistry and Technology of the Silicones], Verlag Chemie GmbH, Weinheim/Bergstraxcex2e, Germany (1968).
Based on the abovementioned components (a), (b) and (c), the proportion of component (b) is usually from 20 to 95% by weight, preferably from 40 to 90% by weight, and particularly preferably from 70 to 90% by weight, expressed as polysiloxane of the formula: RxSiO(2xe2x88x920.5x) which is formed in the condensation.
The silanes of the general formula (I) used according to the invention may be employed wholly or partially in the form of precondensates, i.e. compounds produced by partial hydrolysis of the silanes of the formula (I), either alone or in a mixture with other hydrolysable compounds. Such oligomers, preferably soluble in the reaction medium, may be straight-chain or cyclic low-molecular-weight partial condensates (polyorgano-siloxanes) having a degree of condensation of e.g. from about 2 to 100, in particular from about 2 to 6.
The amount of water employed for hydrolysis and condensation of the silanes of the formula (I) is preferably from 0.1 to 0.9 mol, and particularly preferably from 0.25 to 0.75 mol, of water per mole of the hydrolysable groups which are present. Particularly good results are often achieved with from 0.35 to 0.45 mol of water per mole of the hydrolysable groups which are present.
Specific examples of colloidal inorganic particles (a) are sols and powders dispersible at the nano level (particle size preferably up to 300 nm, in particular up to 100 nm and particularly preferably up to 50 nm) of SiO2, TiO2, ZrO2, Al2O3, Y2O3, CeO2, SnO2, ZnO, iron oxides or carbon (carbon black and graphite), in particular of SiO2.
The proportion of component (a), based on the components (a), (b) and (c), is usually from 5 to 60% by weight, preferably from 10 to 40% by weight, and particularly preferably from 10 to 20% by weight.
For preparing the nanocomposite, other additives in amounts of up to 20% by weight, preferably up to 10% by weight, and in particular up to 5% by weight, may be employed as optional components (c); examples are curing catalysts, such as metal salts and metal alkoxides (e.g. aluminium alkoxides, titanium alkoxides or zirconium alkoxides), organic binders, such as polyvinyl alcohol, polyvinyl acetate, starch, polyethylene glycol and gum arabic, pigments, dyes, flame retardants, compounds of glass-forming elements (e.g. boric acid, boric acid esters, sodium methoxide, potassium acetate, aluminium sec-butoxide, etc).
The hydrolysis and condensation is carried out under sol-gel conditions in the presence of acid condensation catalysts (e.g. hydrochloric acid) at a pH of preferably from 1 to 2, until a viscous sol is produced.
It is preferable if no additional solvent is used besides the solvent produced in the hydrolysis of the alkoxy groups. If desired, however, alcoholic solvents, such as ethanol, or other polar, protic or aprotic solvents, such as tetrahydrofuran, dioxane, dimethylformamide or butyl glycol, for example, may be employed.
In order to achieve a favourable sol particle morphology and sol viscosity, the resultant nanocomposite sol is preferably subjected to a special post-reaction step in which the reaction mixture is heated to temperatures of from 40 to 120xc2x0 C. over a period of from a number of hours to a number of days. Special preference is given to storage for one day at room temperature or heating for a number of hours at from 60 to 80xc2x0 C. This gives a nanocomposite sol with a viscosity of preferably from 5 to 500 mpas, particularly preferably from 10 to 50 mPas. The viscosity of the sol can also, of course, be adjusted to suitable values for the specific application by adding solvents or removing side-products of the reaction (e.g. alcohols). The post-reaction step may preferably also be coupled with a reduction of the solvent content.
The nanocomposite sol and the foundry sand are combined after at least initial hydrolysis of component (b) and in any case before final curing. Before it is brought into contact with the sand, the nanocomposite sol is preferably activated by feeding in a further amount of water.
For the production of foundry molds and cores, the nanocomposite sol is admixed with the foundry mold or core sand in the usual amounts, e.g. in an amount of from 0.1 to 20% by weight.
Additionally, conventional foundry additives may be used, if desired, such as, e.g., solidification oils, core oils, release agents or conventional core binders.
The curing may be carried out at room temperature, although a heat treatment at temperatures of above 50xc2x0 C., preferably above 100xc2x0 C., and more preferably at 150xc2x0 C. or above, is preferred. Curing may, optionally, be carried out in an inert gas atmosphere.
It is found that in comparison to conventionally bonded cores a significantly lower amount of off-gas is emitted and that the mold can be freed from sandy deposits by means of the conventional standard procedures. Furthermore, a significantly smaller gas blast was observed during the casting operation, which is of decisive importance in practice since thereby a higher surface quality can be achieved and finer structures can be cast.
The following examples further illustrate the present invention.